package/xen: force location of init scripts

package/xen: force location of init scripts

The Xen configure scripts looks at the build host to decide where to
install the Sys-V startup scripts, and that location differs between
various distros.

Force the location.

Fixes:
    http://autobuild.buildroot.org/results/869/869829ab086e824d164c5c5ec7f087ed83993be6/
    http://autobuild.buildroot.org/results/336/3360e5a9e3d007b4ed77345b5fe93b2dacb6ad49/
    http://autobuild.buildroot.org/results/29e/29e308ce3cc9c83497ba1c1f98fcda3f48fd03c4/

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Matt Weber <matthew.weber@rockwellcollins.com>
Cc: Alistair Francis <alistair@alistair23.me>
Reviewed-by: Alistair Francis <alistair@alistair23.me>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>